Which microorganisms are plausible identities for those found in hot springs?

Thermophilic Archaea, such as Thermus or Pyrococcus, are specifically adapted to thrive in extreme environments, including the high temperatures found in hot springs. These organisms possess unique biochemical and physiological traits that allow them to maintain stability and functionality of their cellular structures and enzymes under extreme heat. For instance, the enzymes of thermophilic Archaea are often more robust and heat-stable, making them of great interest in various industrial applications such as biotechnology and molecular biology.

In contrast, while bacteria can also be found in hot springs, not all bacterial species are thermophilic; only certain types are capable of surviving in these extreme conditions. Fungi and protozoa generally prefer milder environments, and their cellular machinery is not typically suited for the high temperatures present in hot springs, which limits their presence in such habitats. Therefore, among the options provided, thermophilic Archaea are the most plausible identities for microorganisms found specifically in hot springs due to their unique adaptations to extreme thermal conditions.